Photovoltaic power conditioning units

ABSTRACT

We describe a photovoltaic (PV) panel power conditioning circuits, in particular for a PV panel with multiple sub-strings of connected solar cells. The power conditioning unit comprises a set of input power converters, one connected to each sub-string, a shared dc link to provide a common dc bus for the set of input power converters, and a common output power conversion stage coupled to the shared dc link to convert power from the shared dc link to ac power for a mains power supply output from the power conditioning unit. Local conversion of the sub-strings facilitates control of the power available from the panel and optimum energy harvesting, as well as local maximum power point tracking (MPPT) adjustment.

CLAIM OF BENEFIT TO PRIOR APPLICATIONS

This application claims priority to an earlier-filed United KingdomPatent Application 1118850.5, filed Nov. 1, 2011, which is incorporatedherein by reference.

FIELD OF THE INVENTION

This invention relates to system architectures, circuits and techniquesfor photovoltaic (PV) power conditioning units. Embodiments of theinvention are particularly useful for addressing problems associatedwith partial shading of a PV panel.

BACKGROUND TO THE INVENTION

In a photovoltaic module (panel) the panel is made up ofseries-connected mono crystalline or polycrystalline solar cells, eachhaving a forward voltage of around 0.6V, that is a forward diode drop.These are then series connected to form sub-strings, nominally of around10V for a 60 cell module, and 12V for a 72 cell module. Under certaincircumstances the solar PV modules can become partly shaded, for examplefrom fallen leafs or from part of a building and no longer generatecurrent or voltage. It is desirable to be able to improve the powerharvest from a PV panel under such circumstances, in particular whilstprotecting the PV panel from damage.

Background prior art can be found in: WO2010/144637; U.S. Pat. No.7,031,176; JP2006-041440; EP2286645A; and US 2009/0080226.

SUMMARY OF THE INVENTION

According to the present invention there is therefore provided aphotovoltaic (PV) power conditioning unit for delivering power from a PVpanel to an ac power supply output, the PV panel comprising a string ofseries-connected solar cells with a least one electrical tap connectionon said string to define a set of series-connected sub-strings of saidsolar cells, the power conditioning unit comprising: a set of inputpower converters, one for each said sub-string, each having a pair of dcinput connections for connection to either end of a said sub-string,each of said input power converters having a power output; a shared dclink to provide a common dc bus for said set of input power converters,wherein said power output of each said input power converter is coupledto said shared dc link to provide power from a said sub-string to saiddc bus; an energy storage capacitor, coupled to said shared dc link, tostore power from said PV panel for delivering to said ac power supplyoutput; and a common output power conversion stage coupled to saidshared dc link to convert dc power from said shared dc link to ac powerto provide an ac output from said power conditioning unit.

The above described system architecture enables a number of advantagesincluding active bypassing to protect cells of a sub-string, andimproved MPPT (maximum power point control) to facilitate optimum energyharvesting from the PV panel.

The power conditioning unit may be provided in a separate moduleconnected to the PV panel or, as described later, it may be integratedwith the panel itself.

In embodiments of the architecture an input power converter is avoltage-increasing dc-to-dc power converter comprising a dc-to-acconverter coupled to a transformer coupled, in turn, to an ac-to-dcconversion stage or full wave rectifier. In some embodiments the dcoutputs of the input power converters are connected together to theshared dc link. In other embodiments the ac outputs of the transformersare connected together to provide a shared ac output which is thenrectified to provide power to the dc link. In this latter case thedc-to-ac converters of the input power converters are synchronised sothat the ac outputs are also synchronised. Alternatively thetransformers of the input power converters may be shared so that eachdc-to-ac converter drives a winding of a shared power couplingtransformer having a common output winding coupled to a rectificationstage and then to the dc link. The dc-to-ac power converter may comprisea resonant switching power converter, for example an LLC resonantconverter with zero voltage switching, but this is not essential andother techniques such as direct duty cycle control may also be employed.

A shaded cell of a PV panel becomes a load and power generated in othersolar cells of the panel may be dissipated in the shaded cell, creatinga local hotspot and resulting in long term degradation and prematurefailure of the panel. This can be addressed by providing diodes tobypass the shaded cells, but this also had disadvantages because thereis a relatively significant forward conduction voltage loss (of order 1volt) and power dissipation which can require heat sinking. Diodefailures can also occur.

In embodiments of the above described invention, therefore, thesub-strings lack bypass diodes and the power conditioning unit includesone or more sensors to sense a voltage and/or current on a sub-string todetect shading of the sub-string, and one or more bypass controllers (abypass controller may be shared between sub-strings) which, in responseto detection of sub-string shading, controls the input power converterconnected to that sub-string to reduce or substantially stop powerconversion from the sub-string. In embodiments where the input powerconverter comprises a dc-to-ac converter comprising a set of switches,for example MOSFETs, connected across the dc input of the powerconverter (for example, in a half-bridge type arrangement), the bypasscontroller controls these switches on to provide a bypass current pathfor the shaded sub-string. In some preferred embodiments the bypasscontroller function is combined with a controller performing MPPT on asub-string as described further below. Optionally the PV module may beprovided with a temperature sensor for the panel on each sub-string, anda bypass controller may then be configured to reduce rather than stoppower conversion from a shaded sub-string so that the shaded sub-stringcan continue to produce power provided that it does not become hotterthan some pre-determined threshold temperature value.

The input power converters may have outputs connected to the dc buseither in parallel or in series. When the outputs are connected inseries the effective shading of a sub-string is to reduce the voltage onthe dc bus and thus in the circumstances the bypass controller (oranother controller of the power conditioning unit) may control theremaining, unshaded input converters to increase that output voltage tocompensate. This may be achieved, for example, by changing the operatingfrequency of a resonant converter such as an LLC converter to pull theconverter off resonance and increase the voltage gain by a factor of 50%(this otherwise being determined by the terms ratio of the transformer).Alternatively a similar result may be achieved by direct duty cyclecontrol of a hard switching converter. In this way the system cancontinue to operate efficiently even where one sub-string is shaded.

In some embodiments of the power conditioning unit the dc-to-acconverters of the input power converters are synchronised to operate inan interleaved manner, each operating at a relative phase offset of180°/n or 360°/n, where n is the number of input power converters. Thus,for example, three input converters may operate at 60° or 120° out ofphase with respect to one another. This can substantially reduce rippleon the dc link, for example by a factor of approximately 6, for threeinput power converters.

Optionally a or each input power converter may comprise two switchingpower converters with their inputs connected in parallel (to asub-string) and their outputs connected in parallel at the dc link. Theneach converter may be designed to operate at less than the maximumdesign load and the converters separately enabled or disabled to operateeither and/or both according to the power transferred to the output.Thus one or other or both of the parallel-connected input converters maybe operated depending upon the power transferred by theparallel-connected converters, to thereby shape the efficiency curve ofthe combined system. To achieve this, the system may incorporate one ormore power level controllers to selectively enable operation of theparallel-connected power converters; this controller may be incorporatedinto an MPPT controller for a sub-string. Optionally the power levelcontroller may be configured to switch between the paralleled powerconverter stages using a soft-switching technique to graduallyenable/disable a converter, for example by frequency modulation and/orduty cycle modulation of the dc-to-ac stages of the power converters.Such techniques are described in more detail in our co-pending UK patentapplication No: GB1104800.6 filed on 22 Mar. 2011, the contents of whichare hereby incorporated by reference in their entirety.

In some preferred implementations the power conditioning unit includes amaximum power point tracking (MPPT) control system. Preferably this isemployed to control the common output power conversion stage to, ineffect, control the power drawn from preceding stages, for example asdescribed in our GB2,478,789, hereby incorporated by reference in itsentirety. However some preferred embodiments of the power conditioningunit additionally include a set of secondary MPPT control loops, one foreach sub-string, operating on the respective input power converters, toimprove MPPT performance under partial shading of a PV panel. Optionallythe MPPT controller for an input converter/sub-string may also beconfigured to identify partial shading of a sub-string. This may beperformed simply by monitoring current and/or voltage output from thesub-string or, in a more sophisticated approach, the shape of an I-Vcurve of a sub-string may be employed to identify shading of thesub-string. Thus whilst an unshaded sub-string may have an I-V curvewhich is convex with a single maximum, a shaded sub-string can departfrom this curve shape, for example exhibiting one or more concavefeatures or indentations which can lead to multiple local I-V maxima onthe curve.

In embodiments the MPPT control system for the output power conversionstage has a sense input coupled to the dc link to sense a value of asinusoidal voltage component at twice a frequency of the ac mains onthis dc link, and is configured to control power injected into the acmains supply from the output power conversion stage in response to alevel of this sinusoidal voltage component. More particularly the outputstage MPPT loop may be configured to control an amplitude of ac currentprovided to the ac mains power supply such that an amount of powertransferred to the ac output is dependent on, more partiallyproportional to, an amplitude of the sinusoidal voltage component on theenergy storage capacitor.

More generally in embodiments the implementation of the controlfunctions of the power conditioning unit may be performed by multipleseparate signal processors or by a separate processor per sub-string, orby one or more shared signal processors. Such a signal processor may beimplemented in hardware, for example on an ASIC (applications specificintegrated circuit) or in software, for example in combination with aDSP (digital signal processor) or firmware stored in non-volatile orread-only memory, or the control functions may be implemented in acombination of hard ware and software/firmware, optionally distributedbetween a plurality of coupled components in communication with oneanother.

Sub-string sensing for local, sub-string MPPT control raises someparticular problems, in part because the sub-strings are, in effect,floating rather than ground-referenced, and in part because the sensedsignals may need amplification.

Thus in a related aspect the invention provides a photovoltaic (PV)power conditioning unit for delivering power from a PV panel to an acpower supply output, the PV panel comprising a string ofseries-connected solar cells with a least one electrical tap connectionon said string to define a set of series-connected sub-strings of saidsolar cells, the power conditioning unit comprising: a sensing circuitto sense a voltage or current from one or more of said solar cellswherein said sensing circuit comprises: a sense input to receive avoltage signal from said one or more solar cells dependent on saidsensed voltage or current; and a voltage-programmed current sourcehaving a current programming input coupled to said sense input, andhaving a programmed current output to output a programmed currentdependent on a level of said voltage signal, and wherein said programmedcurrent output is coupled to a current-to-voltage converter to convertsaid programmed current to a voltage output from the sensing circuitdependent on said programmed current.

The voltage input to the sub-string sensing system may either be avoltage produced by the sub-string or a voltage across a current-sensingresistor to sense a current produced by the sub-string. In embodimentsthis sensing system facilitates sensing a ‘floating’ voltage as well asfacilitating the amplification of small signals without the use ofoperational amplifiers and the like. The sensed voltage and/or currentfrom a sub-string may be employed for controlling the power conditioningunit, for example for an MPPT control loop and/or a bypass function.

In some preferred embodiments the voltage input of the sensing circuitis provided to a full-wave rectification circuit such as a bridgerectifier to provide a by-polar (polarity-insensitive) input prior todriving the voltage-programmed constant current generator (which may beeither a current source or a current sink). Preferably the programmablecurrent generator provides an input to a current mirror which has one ormore, optionally amplified, current outputs to provide an output currentproportional to the voltage (or current) of the sensed sub-string. Theoutput current may then be provided to the current-to-voltage converter,which may in embodiments simply comprise a resistor.

As previously described, embodiments of the above described architectureprovide multiple input power converters, one for each sub-string of a PVpanel. Also, optionally, each of these input power converters maycomprise a set of parallel-connected inverters in an ‘interleaved’configuration in which either or both may be selected for operationaccording to the power transferred, to shape the efficiency curve of acombined system. In our related patent application GB1104800.6 (ibid) wealso describe techniques in which, independently of whether or not asub-string power conversion is employed, multiple front end converterstages are connected in parallel and separately enabled/disableaccording to the power transferred.

The inventors have recognised that these system architectures may becombined into a flexible architecture which may be configured for eithersub-string power conversion or switched front end power conversion.

Thus in a related aspect the invention provides a multipurposephotovoltaic power conditioning unit circuit board, for delivering powerfrom a PV panel to an ac power supply output, the PV panel comprising astring of series-connected solar cells with a least one electrical tapconnection on said string to define a set of series-connectedsub-strings of said solar cells, the circuit board comprising: a set ofinput power converters, one for each said sub-string, each having a pairof dc input connections for connection to either end of a saidsub-string, each of said input power converters having a power output; ashared dc link to provide a common dc bus for said set of input powerconverters, wherein said power output of each said input power converteris coupled to said shared dc link to provide power from a saidsub-string to said dc bus; a common output power conversion stagecoupled to said shared dc link to convert dc power from said shared dclink to ac power to provide an ac output from said power conditioningunit; and at least one controller for said power conditioning unit;wherein said at least one controller is configurable to control saidpower conditioning unit in a first sub-string converter mode in whicheach said input power converter receives power from a respective saidsub-string, and in a second mode in which said set of input powerconverters is connected in parallel.

In embodiments in the first mode the controller is configured to provideMPPT and/or bypass functions for each sub-string of the PV panel. In thesecond mode the controller is configured to selectively enable operationof one or more of the input power converters dependent on the level ofpower drawn from the PV panel. The switching between these twoconfigurations may be performed in hardware, for example by means of alink, software, or firmware, for example a configuration setting inEPROM. In embodiments, for flexibility, each input power converter has apair of inputs and each pair of inputs in provided with a connectionfrom the power conditioning unit circuit board. In this way the inputpower converters may be, for example, or connected in parallel to asingle PV panel or each connected to a sub-string of a PV panel.

In a further related aspect the invention provides a photovoltaic powerconditioning unit for extracting power from a PV panel and providing anac output, the PV panel comprising a set of series-connected sub-stringsof solar cells, the power conditioning unit comprising: at least onepower converter to extract power from a said sub-string for said acoutput; a sensing system to detect that a current source representingsaid sub-string is starting to reverse; and a current bypass systemcoupled to said sensing system to, responsive to said detection, bypasssaid sub-string in which said current source is starting to reverse.

Preferably the power conditioning unit comprises a set of the powerconverters each to provide power from a respective sub-string to the acoutput, each having a respective sensing system and current bypasssystem. In embodiments the current bypass system comprises one or moresemi-conductor switching devices connected across the input of a powerconverter that is across the sub-string to which the power converter isconnected in use. Preferably each power converter/sub-string also isassociated with an MPPT controller to maximise the power extracted fromthe respective sub-string.

In this way, broadly speaking, the photovoltaic power conditioning unitis configured to extract as much as possible of the power per sub-stringto the point where the current source will reverse; and at this pointthe sub-string is bypassed to, effectively, re-circulate this energy.This approach provides much improved control over part of the panelbehaviour.

In a related aspect the invention also provides a method of extractingpower from a PV panel, the PV panel comprising a set of series-connectedsub-strings of solar cells, the method comprising: extracting power froma said sub-string until detecting that a current source representingsaid sub-string begins to reverse; and bypassing said sub-stringresponsive to said detecting.

In a further related aspect the invention provides a photovoltaic (PV)power conditioning unit for delivering power from one or more PV panelsto an ac power supply output, the power conditioning unit comprising: aset of input power converters, each having a pair of dc inputconnections and a power output; a shared dc link to provide a common dcbus for said set of input power converters, wherein said power output ofeach said input power converter is coupled to said shared dc link toprovide power from said one or more PV panels to said dc bus; and acommon output power conversion stage coupled to said shared dc link toconvert dc power from said shared dc link to ac power to provide anoutput from said power conditioning unit.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described,by way of example only, with reference to the accompanying figures, inwhich:

FIG. 1 shows an outline block diagram of an example power conditioningunit;

FIGS. 2 a and 2 b show details of a power conditioning unit of the typeshown in FIG. 1;

FIGS. 3 a and 3 b show details of a further example of solarphotovoltaic inverter in which an input power converter incorporates anLLC resonant power converter;

FIGS. 4 a to 4 d show, respectively, a power conditioning circuitcomprising multiple parallel boost/isolation stages according toembodiment of an aspect of the invention, a more detailed circuitdiagram for portion of the power conditioning unit of FIG. 6 a, a powerconditioning unit including multiple parallel boost/isolation stageseach comprising a transformer with an integrated output inductoraccording to an embodiment of the invention, and waveforms illustratingexamples of switching phase offset in a power conditioning unitcomprising parallel boost/isolation stages using an LLC conversiontopology;

FIGS. 5 a and 5 b show example output current-voltage characteristic ofphotovoltaic panels indicating the locations of a maximum output powerpoint;

FIGS. 6 a and 6 b show, respectively, a block diagram of an example dcinput portion and a block diagram of an example ac output portion of aphotovoltaic power conditioning unit incorporating an MPPT trackingsystem;

FIGS. 7 a and 7 b show, respectively, a circuit diagram of an example dcinput portion and of an example ac output portion of a photovoltaicpower conditioning unit incorporating an MPPT tracking system;

FIG. 8 shows the voltage on a DC link capacitor voltage in aphotovoltaic power conditioning unit incorporating an MPPT tracking;

FIG. 9 shows an example control procedure for the power injectioncontrol block of the power conditioning unit of FIG. 6;

FIGS. 10 a and 10 b show, respectively, an example internal circuit of aPV panel, illustrating bypass diodes, and an equivalent electricalcircuit for a photovoltaic cell;

FIGS. 11 a and 11 b show, respectively, first and second example systemarchitectures for photovoltaic panel/inverter systems according toembodiments of the invention;

FIGS. 12 a and 12 b show, respectively, an example implementation of aninput power conversion stage and of a common grid interface stage, forthe architectures of FIG. 11;

FIGS. 13 a to 13 c show, respectively, an example circuit implementationof the input power conversion stage of a power conditioning unitaccording to an embodiment of the invention, and first and secondvariants of the circuit architecture of FIG. 13 a;

FIG. 14 shows a variant circuit architecture to the arrangements of FIG.13 with an ‘interleaved’ topology;

FIG. 15 shows a variant circuit architecture to the arrangements forFIG. 13 including sub-string MPPT and bypass control;

FIGS. 16 a and 16 b show circuit architectures employingvoltage-frequency based sub-string MPPT/dc gain control;

FIG. 17 shows a circuit architecture employing direct duty cyclesub-string MPPT control;

FIGS. 18 a to 18 c show example circuit architectures without bypassdiodes, employing active bypass control;

FIGS. 19 a to 19 c show, respectively, first and second sub-stringsensing circuit architectures, and an embodiment of a sub-string sensingcircuit;

FIG. 20 shows a schematic block diagram of a multipurpose photovoltaicpower conversion unit circuit board; and

FIG. 21 shows a vertical cross section through an integratedphotovoltaic panel and power conditioning unit.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS Power Conditioning Units

By way of background, we first describe an example photovoltaic powerconditioning unit. Thus FIG. 1 shows photovoltaic power conditioningunit of the type we described in WO2007/080429. The power converter 1 ismade of three major elements: a power converter stage A, 3, a reservoir(dc link) capacitor C_(dc) 4, and a power converter stage B, 5. Theapparatus has an input connected to a direct current (dc) power source2, such as a solar or photovoltaic panel array (which may comprise oneor more dc sources connected in series and/or in parallel). Theapparatus also has an output to the grid main electricity supply 6 sothat the energy extracted from the dc source is transferred into thesupply. Capacitor C_(dc) is preferably non-electrolytic, for example afilm capacitor.

The power converter stage A may be, for example, a step-down converter,a step-up converter, or it may both amplify and attenuate the inputvoltage. In addition, it generally provides electrical isolation bymeans of a transformer or a coupled inductor. In general the electricalconditioning of the input voltage should be such that the voltage acrossthe dc link capacitor C_(dc) is always higher than the grid voltage. Ingeneral this block contains one or more transistors, inductors, andcapacitors. The transistor(s) may be driven by a pulse width modulation(PWM) generator. The PWM signal(s) have variable duty cycle, that is,the ON time is variable with respect to the period of the signal. Thisvariation of the duty cycle effectively controls the amount of powertransferred across the power converter stage A.

The power converter stage B injects current into the electricity supplyand the topology of this stage generally utilises some means to controlthe current flowing from the capacitor C_(dc) into the mains. Thecircuit topology may be either a voltage source inverter or a currentsource inverter.

FIG. 2 shows details of an example of a power conditioning unit of thetype shown in FIG. 1; like elements are indicated by like referencenumerals. In FIG. 2 a Q1-Q4, D1-D4 and the transformer form a dc-to-dcconversion stage, here a voltage amplifier. In alternative arrangementsonly two transistors may be used; and/or a centre-tapped transformerwith two back-to-back diodes may be used as the bridge circuit.

In the dc-to-ac converter stage, Q9, D5, D6 and Lout perform currentshaping. In alternative arrangements this function may be located in aconnection between the bridge circuit and the dc link capacitor: D₆ actsas a free-wheeling diode and D₅ prevents current form flowing back intothe dc-link. When transistor Q₉ is switched on, a current builds upthrough L_(out). When Q₉ is switched off, this current cannot return tozero immediately so D₆ provides an alternative path for current to flowfrom the negative supply rail (D₅ prevents a current flowing back intothe dc-link via the body diode in Q₉ when Q₉ is switched off). Currentinjection into the grid is controlled using Q₉: when Q₉ is turned on thecurrent flowing through L_(out) increases and decreases when it isturned off (as long as the dc-link voltage is maintained higher than thegrid voltage magnitude). Hence the current is forced to follow arectified sinusoid which is in turn unfolded by the full-bridge output(transistors Q₅ to Q₈). Information from an output current sensor isused to feedback the instantaneous current value to a control circuit.The inductor current, i_(out), is compared to a reference current,i_(ref), to determine whether or not to switch on transistor Q₉. If thereference current is higher than i_(out) then the transistor is turnedon; it is switched off otherwise. The reference current, i_(ref), may begenerated from a rectified sinusoidal template in synchronism with theac mains (grid) voltage.

Transistors Q5-Q8 constitutes an “unfolding” stage. Thus thesetransistors Q5-Q8 form a full-bridge that switches at line frequencyusing an analogue circuit synchronised with the grid voltage.Transistors Q5 and Q8 are on during the positive half cycle of the gridvoltage and Q6 and Q7 are on during the negative half cycle of the gridvoltage.

Thus in embodiments the power conditioning unit comprises a genericdc-ac-dc that provides voltage amplification of the source to above thegrid voltage, and isolation, and a current source inverter (CSI)connected to the mains. The current injection is regulated using currentshaping (current-control) in the inductor of the CSI via theintermediate buck-type stage. (This is described further in ourGB2415841B, incorporated by reference).

Control (block) A of FIG. 1 may be connected to the control connections(e.g. gates or bases) of transistors in power converter stage A tocontrol the transfer of power from the dc energy source. The input ofthis stage is connected to the dc energy source and the output of thisstage is connected to the dc link capacitor. This capacitor storesenergy from the dc energy source for delivery to the mains supply.Control (block) A may be configured to draw such that the unit drawssubstantially constant power from the dc energy source regardless of thedc link voltage V_(dc) on C_(dc).

Control (block) B may be connected to the control connections oftransistors in the power converter stage B to control the transfer ofpower to the mains supply. The input of this stage is connected to thedc link capacitor and the output of this stage is connected to the mainssupply. Control B may be configured to inject a substantially sinusoidalcurrent into the mains supply regardless of the dc link voltage V_(dc)on C_(dc).

The capacitor C_(dc) acts as an energy buffer from the input to theoutput. Energy is supplied into the capacitor via the power stage A atthe same time that energy is extracted from the capacitor via the powerstage B. The system provides a control method that balances the averageenergy transfer and allows a voltage fluctuation, resulting from theinjection of ac power into the mains, superimposed onto the average dcvoltage of the capacitor C_(dc). The frequency of the oscillation can beeither 100 Hz or 120 Hz depending on the line voltage frequency (50 Hzor 60 Hz respectively).

Two control blocks control the system: control block A controls thepower stage A, and control block B power stage B. An exampleimplementation of control blocks A and B is shown in FIG. 2 b. In thisexample these blocks operate independently but share a commonmicrocontroller for simplicity.

In broad terms, control block A senses the dc input voltage (and/orcurrent) and provides a PWM waveform to control the transistors of powerstage A to control the power transferred across this power stage.Control block B senses the output current (and voltage) and controls thetransistors of power stage B to control the power transferred to themains. Many different control strategies are possible. For exampledetails of one preferred strategy reference may be made to our earlierfiled WO2007/080429 (which senses the (ripple) voltage on the dclink)—but the embodiments of the invention we describe later do not relyon use of any particular control strategy.

In a photovoltaic power conditioning unit the microcontroller of FIG. 2b will generally implement an algorithm for some form of maximum powerpoint tracking. In embodiments of the invention we describe later thisor a similar microcontroller may be further configured to controlwhether one or both of the dc-to-dc power converter stages areoperational, and to implement “soft” switching off of one of thesestages when required. The microcontroller and/or associated hardware mayalso be configured to interleave the power transistor switching,preferable to reduce ripple as previously mentioned.

Now referring to FIG. 3 a, this shows a further example of a powerconditioning unit 600. In the architecture of FIG. 3 a photovoltaicmodule 602 provides a dc power source for dc-to-dc power conversionstage 604, in this example each comprising an LLC resonant converter.Thus power conversion stage 604 comprises a dc-to-ac (switching)converter stage 606 to convert dc from module 602 to ac for atransformer 608. The secondary side of transformer 608 is coupled to arectifying circuit 610, which in turn provides a dc output to aseries-coupled output inductor 612. Output inductor 612 is coupled to adc link 614 of the power conditioning unit, to which is also coupled adc link capacitor 616. A dc-to-ac converter 618 has a dc input from a dclink and provides an ac output 620, for example to an ac grid mainssupply.

A microcontroller 622 provides switching control signals to dc-to-acconverter 606, to rectifying circuit 610 (for synchronous rectifiers),and to dc-to-ac converter 618 in the output ‘unfolding’ stage. Asillustrated microcontroller 622 also senses the output voltage/currentto the grid, the input voltage/current from the PV module 602, and, inembodiments, the dc link voltage. (The skilled person will be aware ofmany ways in which such sensing may be performed). In some embodimentsthe microcontroller 622 implements a control strategy as previouslydescribed. As illustrated, the microcontroller 622 is coupled to an RFtransceiver 624 such as a ZigBee™ transceiver, which is provided with anantenna 626 for monitoring and control of the power conditioning unit600.

Referring now to FIG. 3 b, this shows details of a portion of an exampleimplementation of the arrangement of FIG. 3 a. This example arrangementemploys a modification of the circuit of FIG. 2 a and like elements tothose of FIG. 2 a are indicated by like reference numerals; likewiselike elements to those of FIG. 3 a are indicated by like referencenumerals. In the arrangement of FIG. 3 b an LLC converter is employed(by contrast with FIG. 2 a), using a pair of resonant capacitors C1, C3.

The circuits of FIGS. 1 to 3 are particularly useful for microinverters,for example having a maximum rate of power of less than 1000 Watts andor connected to a small number of PV modules, for example just one ortwo such modules. In such systems the panel voltages can be as low as 20volts and hence the conversion currents can be in excess of 30 amps RMS.

Interleaved Topology

The output dc-to-ac converter stage may be implemented in any convenientmanner. However embodiments of the photovoltaic power conditioning unitmay employ multiple front end power converter stages connected inparallel between the dc input of the power conditioning unit and the dclink. In embodiments each of these power converter stages implements aboost/isolation stage using an LLC (resonant) conversion topology, witheach with a transformer and an output inductance (which may beintegrated with the transformer). This helps reduce a significant sourceof power losses (ac losses and I²R losses) in the initialboost/isolation stage, between the dc input and dc link of the powerconditioning unit. Further details may be found in our co-pending patentapplication GB1104800.6 filed 22 Mar. 2011.

Each front end converter stage may operate substantially independentlyin the sense that it may be separately enabled or disabled, to therebyshape the efficiency curve of the combined system. This is particularlybeneficial in a system which is operating at less than 100% of itsavailable power (rating). Soft switching of a power converter may beemployed during enable/disable of a converter, for example by frequencymodulation and/or duty cycle modulation. Embodiments also use phaseoffset switching of the converter stages to minimise ac ripple currentlosses as well as dc losses. Each converter stage may be designed tooperate at 50% of the maximum load (plus some additional margin); outputinductance may be used to enforce 50/50 sharing (though this is notessential).

Thus referring now to FIG. 4 a, this shows a first embodiment of a powerconditioning unit 600 according to an aspect of the invention. In thearchitecture of FIG. 4 a photovoltaic module 602 provides a dc powersource for first and second dc-to-dc power conversion stages 604 a, b,in this example each comprising an LLC resonant converter. Thus each ofpower conversion stages 604 comprises a dc-to-ac (switching) converterstage 606 a, b to convert dc from module 602 to ac for a respectivetransformer 608 a, b. The secondary side of transformers 608 a, b arecoupled to respective rectifying circuits 610 a, b, which in turnprovide a dc output to a respective series-coupled output inductor 612a, b. Each of output inductors 612 a, b is coupled to a dc link 614 ofthe power conditioning unit, to which is also coupled a dc linkcapacitor 616. A dc-to-ac converter 618 has a dc input from a dc linkand provides an ac output 620, for example to an ac grid mains supply.

A microcontroller 622 provides switching control signals to dc-to-acconverters 606 a, b, to rectifying circuits 610 a, b (for synchronousrectifiers), and to dc-to-ac converter 618 in the output ‘unfolding’stage. As illustrated microcontroller 622 also senses the outputvoltage/current to the grid, the input voltage/current from the PVmodule 602, and, in embodiments, the dc link voltage. In some preferredembodiments the microcontroller 622 implements a control strategy aspreviously described, although the operation of embodiments of theinvention is not tied to any particular control strategy or, forexample, to any particular MPPT (maximum power point tracking) strategy.

In the circuit of FIG. 4 a the output inductors 612 a,b effectivelyforce load sharing between the front end converters 604 a,b. (We referto the addition of the individual chokes in the output circuit asproportional load sharing). Efficiency gains arise because LLCconverters are core-loss dominant, so a smaller core enabled by areduced power rating for each individual converter reduces the overallcore losses. Furthermore if the power of each converter falls by afactor of 2, the I²R losses fall by a factor of 4 (per Watt). Thesetechniques are particularly useful for microinverters, for example witha maximum rate of power of less than 1000 Watts and/or connected to justone or two PV modules. This is because in such systems the panelvoltages can be as low as 20 volts and hence the conversion currents canbe in excess of 30 amps RMS.

Referring now to FIG. 4 b, this shows details of a portion of an exampleimplementation of the arrangement of FIG. 4 a. This example arrangementemploys a modification of the circuit of FIG. 2 a and like elements tothose of FIG. 2 a are indicated by like reference numerals; likewiselike elements to those of FIG. 4 a are indicated by like referencenumerals. In the arrangement of FIG. 4 b an LLC converter is employed(by contrast with FIG. 2 a), using a pair of resonant capacitors C1, C3.FIG. 4 b illustrates ripple current sensing to sense the available powerfrom the photovoltaic module. As illustrated a circuit 622 rectifies aripple voltage across one or both of the resonant capacitors andprovides an output, for example to an analogue-to-digital converter forinterfacing with microcontroller 622. The available power is dependentupon the level of ripple, and the illustrated arrangement provides anefficient way of measuring available power from the panel.

Referring now to FIG. 4 c, this shows a second embodiment of the powerconditioning unit 650 similar to that of FIG. 4 a but with an improvedarrangement of output inductors. More particularly the output inductors612 of the FIG. 4 a are incorporated into respective transformers 652 a,b of the front end dc-to-dc converter stages to facilitate load sharingbetween the conversion stages.

FIG. 4 d illustrates example waveforms of multiphase interleaving of theswitching of the converters 604 a,b (in the Figure the waveformsillustrate example data control signals of the switches in stages 606 a,b of the converters). To reduce ripple on the input capacitor theswitching is preferably 180° out of phase between the two converters.However in embodiments the rectification circuits 610 a,b of the powerconverters may be shared (not shown), i.e. so that a common set ofrectifiers/rectification circuits is employed for both converters. Inthis case the interleaving between the dc-to-ac conversion portions ofthe dc-to-dc converters 604 a,b may be interleaved 90° out of phase, asillustrated in FIG. 4 d. This provides further efficiencies in circuitsimplification, albeit at the expense of increased ripple.

At low input/output powers it is more efficient to run just a singlefront end converter 604, to reduce core loses in the transformer, but athigher input/output powers it is more efficient to run both converters,to reduce resistive losses. To avoid rapid dumping of the energy storedin a transformer 608 preferably microcontroller 622 is configured toswitch a converter on/off gradually, for example either by graduallymoving the switching frequency off resonance and/or by graduallyreducing the duty cycle of a PWM control signal to the converter switch,to reduce the switch on-time and thus gradually dissipate the storedenergy. Such techniques may also be employed to turn a converterpartially on or partially off. The point at which a change-over occursbetween running one converter and running both converters is bestdetermined by experiment, for example by plotting curves of efficiencyand/or loss when running two converters and when just running a singleconverter, to determine the change-over point. Optionally the switchingpoint may be biased towards either single or dual converter use (forexample in the latter case to reduce overall component stresses andhence potentially prolong lifetime), and/or arranged to provide somehysteresis in the switching.

MPPT (Maximum Power Point Tracking) Techniques

Our preferred implementation of maximum power point tracking (MPPT) foruse with the above described solar photovoltaic system architecturecomprises a power injection control block with a sense input coupled tothe (non-electrolytic) energy storage capacitor and an output coupled tocontrol the dc-to-ac converter, to control power injected into the acmains without needing to measure a dc voltage or dc current providedfrom said dc power source. This arrangement may also be used inembodiments of the invention we describe later, but preferably also withadditional, sub-string MPPT tracking loops.

As previously described in embodiments of our preferred architecture avoltage on the energy storage capacitor has a sinusoidal voltagecomponent (at twice the frequency of the ac mains), and the powerinjection control block controls an amplitude of an ac current providedto the ac mains power supply output such that an amount of powertransferred to the output is dependent on an amplitude of the sinusoidalvoltage component on the energy storage capacitor. In embodiments theaverage energy transferred is linearly dependent on, more particularlyproportional to, a squared value of the sinusoidal voltage component.The sinusoidal voltage component is superimposed on a dc link voltage(input to the dc-to-ac converter), and this link voltage is relativelyhigh, for example on average greater than 200, 300, or 400 volts. Theaverage power transferred is proportional to the difference between thepeak (maximum) capacitor voltage squared and the trough (minimum)capacitor voltage squared (although the unit may alternatively bearranged such that there is, on average, zero dc voltage on the energystorage capacitor). The instantaneous power transferred to the ac mainspower supply output is proportional to the instantaneous value ofvoltage on the energy storage capacitor.

In embodiments the method/system comprises two independent controlblocks. The first block controls the voltage amplification stage thatinterfaces with the energy generator. The energy generator is preferablya solar module. In embodiments the first control block does not functionto regulate the amount of energy to be transmitted but functions only asa switch, either allowing energy flow or preventing any energy flow fromthe generator and through the amplification stage, regardless of theamount. The output of the voltage amplification stage is coupled to anenergy reservoir capacitor. Energy flow is therefore dependent on theamount of “room” (the amount of additional energy which can be stored)in the reservoir capacitor. The second control block is a feedbackcontrol loop that interfaces the energy reservoir capacitor to thecoupled output load. The second control block regulates the amount ofpower to be injected into the load by emptying the energy reservoircapacitor. The second control block uses, in embodiments exclusively,the level of voltage fluctuations on the energy reservoir (storagecapacitor) to control the amount of power being extracted from theenergy generator and also the amount of power being injected into theload. In embodiments no use of (measured) current values is made. Thusin embodiments the maximum power point tracking uses two completelyindependent loops and uses exclusively variations characteristic of thereservoir capacitor.

Energy generators such as solar photovoltaic cells have a non-linearpower characteristic profile, such as those illustrated in FIG. 5. InFIG. 5 a maximum power is harvestable at the point labelled X, whichexhibits maximum power point current Imp and voltage Vmp. It ispreferable that the operating point that yields most energy is attainedand maintained.

Referring to FIGS. 6 a and 6 b, these show a block diagram of input 1002and output 1004 stages of an embodiment of a solar PV power conditioningsystem 1000. Thus FIG. 6 a shows an energy generator 1010 such as one ormore PV panels feeding a voltage amplification stage 1012 with asubstantially constant amplification factor (which may be less than,equal to, or greater than unity depending, for example, on whether thedc input is from a single PV panel or a string of series connectedpanels). This in turn provides power to an energy reservoir 1014, inembodiments a storage capacitor coupled to a dc link between the input,voltage amplification stage and an output, voltage inversion stage. InFIG. 6 a control block A 1016 controls voltage amplification stage 1012,but only to switch power from the energy generator on and off into theenergy reservoir: it does not provide a variable gain control and simplycomprises a fixed frequency oscillator. Voltage inversion stage 1018 hasan input coupled to the energy reservoir 1014 and provides an ac mainsoutput to load 1020, for example via a grid connection. Control Block B1022 monitors the voltage on the dc link via sense connection 1022 a(but in embodiments does not sense the current on this link), and thecurrent into and voltage on the load via sense connections 1022 b,c (inembodiments connection 1022 c is within the power conditioning unit),and provides gate drive output signals 1022 d for controlling thevoltage inversion (“unfolding”) stage 1018, more particularly forcontrolling the power drawn from the energy reservoir and provided intothe load via the grid. The gate drive signals 1022 d are sequenced tocontrol the power converter switches of the power conversion stage 1018(see also FIG. 2); this provides a convenient technique for controllingthe switching frequency of this stage. Thus here control block Afunctions as a power switch, allowing power to flow from the energygenerator to the voltage amplification stage (or effectively switchingthe voltage amplification stage on/off or in/out); it can also be set toturn off power from the energy generator in the event, for example ofunder/over-voltage conditions.

The voltage amplification stage can have a fixed or variableamplification ratio; it may comprise a half-bridge, a full bridge, apush-pull or a similar voltage inversion stage coupled to a transformer,whose amplification ratio results in a desired voltage in the DC linkreservoir capacitor, for example of order 400 volts. The output of thetransformer is coupled to a rectifier stage. An inductor may be includedbetween the rectifier bridge and the DC link reservoir capacitor.

FIG. 7 a shows a more detailed circuit diagram of an example input stage1002. The voltage amplification stage comprises a half-bridge, which inturn is made up of two series switches (MOSFETs), Q1 and Q2, and twoseries capacitors C1 and C2, and the transformer TX1. A rectifier bridge1013 made up of diodes is coupled to the output of the transformer andto the DC link capacitor Cd via a filter inductor Ld. The control blockin FIG. 7 a produces a constant duty cycle PWM signal, and hence nomodulation is implemented. In the event that Cd is full, defined as thevoltage across it being equal or larger than the rectified output fromtransformer secondary, no power flows into Cd even though Q1 and Q2 areswitched on and off continuously. Hence in this arrangement controlblock A does not regulate the amount of power extracted from thegenerator.

FIG. 7 b shows a more detailed circuit diagram of an example outputstage 1004 implementing “back-end” MPPT control methodology: controlblock B measures the voltage fluctuations in the DC link that are usedfor regulation of the amount of power being harvested from the energygenerator and therefore the amount of power injected into the load. Apreferred load is the utility grid—in the case of the grid load, controlB measures the peak and trough voltages on the DC link capacitor via ascaling circuit (the potential divider circuit of R3 and R4). The scaledvalues of the peak Vp and the trough Vt voltages are used to compute theamount of power flowing through the capacitor (as described below). Inembodiments the voltage sense connection to Control Block B is via arectifier.

Due to the AC nature of the power being transferred into the grid andthe current-voltage characteristic of the power being generated by thesolar module, energy storage is essential in a PV inverter if maximumpower is to be harvested from the solar module. In our preferreddescribed above architecture energy storage is delegated to the DC linkreservoir capacitor. The amount of power transferred into the grid isrelated to the energy change in the capacitor and therefore the voltageripple on the capacitor. Implementing energy storage on the DC linkallows a large ripple on the capacitor. Equation 1 illustrates therelationship between energy change, the capacitance and the voltage onthe capacitor:

$\begin{matrix}{U_{R} = {\frac{1}{2}{C_{d\; c}\left( {V_{P}^{2} - V_{T}^{2}} \right)}}} & (1)\end{matrix}$

where V_(P) is the capacitor peak voltage and V_(T) is the capacitortrough voltage. The power transferred would be the energy change persecond. FIG. 8 illustrates the fluctuation (sinusoidal ripple) on the DClink capacitor. Control block B automatically achieves MPPT byregulating the amount of injected current with reference to (dependenton) the dc link voltage fluctuation. However this MPPT trackingtechnology is not restricted to a power conditioning unit whichdeliberately allows (and controls based on) a degree of ac ripple on thedc link.

Consider an input current and voltage I, V to the inverter provided by aphotovoltaic power source, a dc link current and voltage I_(d), V_(d),and an output current and voltage into grid mains of I_(grid), V_(grid).Since V_(grid) is approximately constant, the power injected into thegrid mains is proportional to I_(grid). Also, absent losses, the inputpower I.V=I_(d). V_(d). and thus I_(d). V_(d) determines the point onthe photovoltaic IV characteristic at which the system operates. Broadlyspeaking the system senses the ripple on V_(d) which, in embodiments,(as described above) is a measure of the power flowing through the dclink. More particularly the system controls the output “unfolding” stage(for example a buck stage converter) to maximise the level (amplitude)of this ripple component on the dc link voltage/energy storagecapacitor, and hence also to maximise the power injected into the acmains. (The skilled person will appreciate that V_(d) on its own doesnot provide a good measure of the power on the dc link).

In one implementation the control block 1022 generates a half sinusoidaltemplate voltage (with an amplitude varying between zero and 3.3 volts)in phase with the grid, for comparison with a (rectified) version of thesensed load current 1022 b. The sensed load voltage 1022 c is used onlyto determine the phase of the ac mains. The amplitude of the template isadjusted dependent on the level of ripple sensed on the energy storagecapacitor/dc link (via line 1022 a). If the template amplitude isgreater than the amplitude of the sensed grid current the switchingfrequency is increased to inject more power into the grid, and viceversa. Thus, broadly speaking, the amplitude of the template is adjusteddependent on the dc link ripple and the output current is controlled tomatch the template amplitude.

FIG. 9 shows an example control procedure for control block B 1022 (manyvariations are possible). Presuming that the procedure begins atstart-up of the inverter, the procedure first initializes the amplitudeof the template signal to an arbitrary, relatively low value, forexample 0.5 volts on the previous 0.0-3.3 volts scale (step S1500). Atthis point the output voltage from the photovoltaic panel is at amaximum and the output current is at substantially zero; the level ofripple on the dc link is also substantially zero. The proceduredetermines the phase of the ac grid mains voltage (S1502) andsynchronises the half-sinusoidal template to the grid. The procedurethen senses the grid current (S1504), for example by sensing the voltageacross a current sense resistor; at start-up this will be approximatelyzero. The procedure then determines an error value E from the differencebetween the template and the sensed grid current (S1506), which atstart-up (continuing the previous example) will be 0.5. The procedurethen determines a switching rate for the voltage inversion stage 1018dependent upon this error, in one example algorithm increasing theswitching rate if E is positive and decreasing the rate if E isnegative. Thus in the present example, at start-up the templateamplitude is greater than that of the sensed grid current so theswitching rate is increased. This increases the current (and hencepower) injected into the ac grid mains, so that the ripple voltage onthe dc link also increases.

At step S1510 the procedure measures the ripple voltage on the dc linkand, at step S1512, adjusts the template amplitude dependent on thismeasurement, increasing the amplitude if the ripple voltage increased,and vice versa. The procedure then loops back to step S1504 to onceagain sense the current being injected into the ac mains. Thus if, say,the error is positive the template amplitude increases so that it isonce again greater than the amplitude of the sensed current injectedinto the grid, and the switching rate of the voltage inversion stage isonce again increased. However if the previous change decreased themeasured ripple voltage (which senses the power drawn from thephotovoltaic panel), then the template amplitude, and hence switchingrate of the voltage inversion stage, is also decreased. In this way thecontrol technique operates to control the output voltage inversion stagesuch that the photovoltaic panel is maintained at substantially itsmaximum output power point. With this arrangement there is no need tomeasure the dc voltage and current from the PV panel.

These MPPT tracking techniques may also be used with other types ofinverter, for example a ‘four-switch’ inverter as described in our U.S.Pat. No. 7,626,834 (in particular if this is provided with a half orfull bridge dc boost stage (with a transformer) at the front end).

Photovoltaic Panels

In photovoltaic (PV) modules or panels the panel is made up of cellseach having a forward voltage of around 0.6V, forward diode drop. Theseare then series connected to form sub-strings, nominally of around 10Vfor a 60 cell module, and 12V for a 72 cell module. Each sub-string isbypassed by another diode device to protect the cell structure when acell or number of cells are shaded.

FIG. 10 a shows an example internal construction of a photovoltaic panel1050, here comprising three sub-strings each comprising a plurality ofsolar cells (diodes) 1052, 1054, 1056, for example three sub-strings of24 diodes or three sub-strings of 18 diodes for a 72 cell solar module.Each sub-string is provided with a respective bypass diode 1052 a, 1054a, 1056 a. The p-n junctions of the diodes in the strings each generatea voltage which may typically be of order 0.5-0.6 volts; the forwardvoltage drop across a bypass diode may be of order 0.5-1.0 volts.

The bypass diode offers an alternate path from the current sourcesformed by the other sub-strings. Failure to bypass can result in longterm degradation of a cell and premature failure of the panel. Inaddition when bypassed the cell will likely cause degradation of theoverall panel and if excessive shading occurs drop out of the inverterdriving the grid.

FIG. 10 b shows the equivalent circuit of a photovoltaic cell. Thecurrent source represents the photocurrent (IL), generated at thejunction region by the photons with energy enough to produce pairs ofelectrons-holes; the diode represents the PN junction with reversesaturation current (ID); Joule losses and leakage currents arerepresented by the currents through the series resistance (RS) and theshunt resistance (RP) respectively.

When the first Kirchoff law is applied to one of the nodes of theequivalent circuit, the current supplied by a cell, at a specifiedtemperature, is given by:

$\begin{matrix}{I = {{I_{L} - I_{D} - I_{P}} = {I_{L} - {I_{0}\left\{ {{\exp\left\lbrack \frac{{\mathbb{e}}\left( {V + {IR}_{S}} \right)}{{NmKT}_{cel}} \right\rbrack} - 1} \right\}} - \frac{V + {IR}_{S}}{R_{P}}}}} & (1)\end{matrix}$where:

-   -   I is the output current    -   IL is the photocurrent    -   ID is the diode current    -   IP is the leakage current    -   I0 is the reverse saturation current    -   N is the number of cells associated in series    -   m is the diode ideality factor, which lies between 1 and 2 for        monocrystalline silicon    -   K is the Boltzman constant    -   Tcel is the cell temperature    -   e is the electron charge    -   V is the terminal voltage    -   RS is the series resistance    -   RP is the shunt resistance        for an association of cells (as in a photovoltaic module), that        is where N is the number of cells connected in series. Most of        the photovoltaic modules available in the market are constituted        by 30 to 36 cells.

Referring back to FIG. 5 b, three points of the curve may behighlighted:

-   -   a) open-circuit: this point is obtained when the terminals of        the module are disconnected. The module presents a voltage        called the “open-circuit voltage” VOC.    -   b) short-circuit: the terminals of the module are connected with        an ideal conductor, through which flows a current called the        “short-circuit current” (ISC). In this situation, the voltage        between module terminals is zero.    -   c) maximum-power: point where the voltage versus current product        is maximum.

FIG. 5 b shows a generic characteristic curve. It can be observed that,from the short circuit, the current presents a slightly descendingbehaviour until it reaches to an “elbow” from where it decreases quicklydown to zero.

Sub-String Power Conversion Systems

We will describe techniques which avoid the need for bypass diodes. Moreparticularly we will describe techniques which perform local conversionof the sub-strings to enable maximum control of the power rangeavailable from the panel; this may be in the order of a single 1 W atthe sub-string level allowing higher yield. In addition this allowslocal MPPT control adjustment within the nominal control range to adjustthe impedance seen by the Inverter locally from each string—the methodswe describe above are effectively an average of all three string controlpoints, which may not be optimum for each string under all operatingconditions. Additionally the loss in the bypass diodes has been theconcern for panel reliability and is by far the highest failure ratemechanism in PV panels. The active bypassing we will describe affordslower power loss and hence higher reliability for the PV system.Further, voltage sensing of each sub-string can be achieved using anactive current source referenced to either a +ve or −ve PV terminal,thus simplifying the control scheme.

System Architecture

A block level diagram for a first example system architecture 1100 isshown in FIG. 11 a. The PV module 1102 is divided into three independentseparate sections 1102 a,b,c and each section is considered as asub-string. The number of sub-strings can vary depending on the numberof solar cells in the module. Effectively each section operates inparallel: Each sub string is connected to a respective power conversionstage 1104 a,b,c—preferably a dc-to-dc converter which may comprise, forexample, a boost circuit, half bridge, full bridge or a flyback circuit.By way of example we describe below a full bridge circuit connected to ahigh frequency transformer. The conversion stages 1102 a,b,c are coupledto a common bus 1105, which may, but need not necessarily be a common dcbus, and this common bus 1105 provides power to a common grid interfacestage 1106.

A simple implementation of a conversion stage 1102 is shown in FIG. 12a. This example comprises four MOSFETs 1110 a-d in a full bridgearrangement, driven by respective level shifting drivers 1108 a,b,controlled by a controller 1114 and driving a transformer 1112. TheMOSFETS may be rated to <100V for interfacing with the PV module; theymay be switched at 50 kHz with resonant switching to minimize theswitching losses. As shown in FIG. 11 a, the outputs of the input stages1104 are connected in parallel to a common secondary stage 1106. Theskilled person will appreciate that, for example, a half bridge may beemployed instead of a full bridge.

The secondary stage 1106 may comprise any of a variety of DC-ACconversion stages. An example, similar to those previously described, isshown in FIG. 12 b. This example comprises a rectification stage 1116,here formed from four diodes, interfaced to a current shaping buck stage1118 which in turn is connected to a inversion stage, for example a fullbridge of MOSFETs 1120 operating at line frequency. There are many otheralternatives to this, for example a single stage H bridge, using highfrequency MOSFETs for current shaping and therefore eliminating the buckstage.

The three (or n depending on size of the PV module) sub-stringconversion stages each include an MPPT controller 1122 a,b,c to performmaximum power point tracking (MPPT). This is achieved by monitoring thecurrent through each sub-string and the voltage across the sub-string.Any of a range of MPPT control techniques may be employed including, butnot limited to, perturb and observe, an incremental conductance method,and a constant voltage method. Additionally or alternatively the MPPTmay be undertaken at the centralised common grid interface stage, usingan MPPT controller 1124, for example as previously described. Preferablytwo levels of MPPT are be employed to optimise the overall output fromthe unit.

The conversion stages 1104 may optionally incorporate a bypass module1126 a,b,c, for example comprising a power semiconductor switchingdevice such as a MOSFET or IGBT switch, connected across a sub-stringand controlled by a controller to turn the switch on when the need forthe bypass function to be implemented is detected. In one approach theneed for a bypass function to be implemented may be determined bydetermining whether there is significant mismatch in the current flow inor voltage across one of sub-strings compared to the others: this may betaken as an indication that there is shading, and depending on the levelof disparity the shaded sub-string may then be bypassed. (This employscommunication between the bypass modules 1126 which may be, for example,via software on one or more microcontrollers).

Alternatively a bypass function may be efficiently implemented using theMOSFETs 1110 and controller 1114 of FIG. 12 a, bypassing a sub-string byswitching the pair of MOSFETs shown in FIG. 12 a on so that there islittle or no current flow in the sub-string.

In a further approach a bypass function may be incorporated into an MPPTcontroller 1122. The need for a bypass function to be implemented maythen be determined by using the sub-string MPPT controller 1122 toperform a (complete) sweep of the IV curve of the sub-string: If morethan one peak is detected this can be taken as an indication of shading.

An advantage of MOSFET (or other active device) switching is that theconduction loss may be significantly reduced as compared to the use ofbypass diodes. Bypass diodes typically have a forward voltage ˜0.5V andMOSFETs can significantly reduce this, thus reducing losses.

In a variant of the above described bypass control functions, ratherthan completely shut down a shaded sub-string the sub-string may be onlypartially shut down, to continue to try to extract as much power fromthe sub-string as possible. For example, the temperature of one or morecells may be measured and the power conditioning unit controlled tooperate the shaded sub-string intermittently below an upper thresholdtemperature or until an upper threshold temperature is reached.

Referring to now FIG. 11 b, this illustrates an alternative architecture1150 in the context of which the above described techniques may also beemployed. Thus in the architecture of FIG. 11 b the DC-DC converterstages 1104 are connected in place of the by-pass diodes so that theinputs of the DC-DC converters are connected in series. In this approachthe switching in the DC-DC converters may be undertaken synchronously toreduce ripple on the dc link. When a sub-string needs to be bypassedthen the MOSFET(s) are left closed to shunt current which wouldotherwise flow through the sub-string and create a hot spot on theshaded cell(s).

For both of the above two approaches three transformers 1112 may beconnected and operate in parallel, or a common transformer link to thecommon grid interface stage 1106 may be employed.

Circuit Implementation

Referring now to FIG. 13 a, this shows a first example circuitimplementation 1300 of the input power conversion stage of a powerconditioning unit according to an embodiment of the invention. Thecircuit comprises a set of input power convertors 1304, 1306, 1308 eachcomprising a respective pair of MOSFET switches 1310 a, b; 1312 a, b;1314 a, b connected across a respective pair of DC input connections1316 a, b; 1318 a, b; 1320 a, b. In the illustrated example DC inputconnections of the input power convertors for adjacent sub-strings areconnected together to form a shared DC input connection for each tap onthe string of solar cells of the PV panel. Thus DC input connections1316 b and 1318 a, and 1318 b and 1320 a are connected together. Thusthe input power conversion stage 1300 has four DC input connectionpoints 1322 a-d, connection points 1322 a, d connecting to either end ofthe complete string of solar cells of the PV panel, and connectionpoints 1322 b, c connecting to taps on the string of solar cellsdefining, in this example three sub-strings of the complete string. Inthis example the DC input connections of the input power convertors areeach provided with a respective bypass diode 1324, 1326, 1328 (eachimplemented as a pair of parallel-connected diodes), which may bemounted on the same circuit board as the power conditioning unit.

Each pair of MOSFETs has a respective gate drive control connection1330, 1332, 1334 for PWM (Pulse Width Modulation) or resonant control ofthe input power conversion stage. (For simplicity the gate drivecontrollers are omitted from the figure). Each input power conversionstage also includes a small, non-electrolytic capacitor (implemented inthis example as three parallel-connected capacitors) 1336, 1338, 1340,although the main energy storage in the power conversion unit isimplemented on the high voltage DC link, as previously described. Eachinput power convertor also includes a current sensing resistor 1342,1344, 1346 to sense a current provided by the respective sub-string towhich the input power convertor is connected. The current supplied by asub-string is sensed by sensing the voltage across these resistors.

Each input power convertor operates to convert the input DC power to ACwhich is provided to a respective transformer 1348, 1350, 1352 followedby an AC-to-DC convertor, as illustrated respective full bridge stages1354, 1356, 1358. The outputs of each full bridge stage are connected inparallel to a common DC link bus 1360 provided with a parallel-connectedenergy storage capacitor 1362. This DC bus provides DC power to asubsequent common output power conversion stage (not shown in thefigure) as previously described, to provide an AC mains output. Atypical voltage from a PV panel is of order 30 volts and thus in thisexample, with three sub-strings, each sub-string generates of order 10volts. In an example embodiment the DC link 1360 operates at around 320volts and each transformer has a 1:32 turns ratio, where the primary mayjust be one or two turns.

FIG. 13 b illustrates a variant of the arrangement of FIG. 13 a in whichthe separate transformers 1348, 1350, 1352 are replaced by a sharedtransformer 1364. FIG. 13 c illustrates a further variant in which theshared transformer 1366 has a shared, common secondary winding providingpower to a shared rectification stage 1368 coupled to DC link 1360.Preferably for these arrangements the switching of the MOSFETs in theinput power convertors is synchronized to be 60° or 120° apart, toreduce the ripple on the DC link.

Referring next to FIG. 14, this shows a variant circuit architecture1400 to the arrangements of FIG. 13, illustrating that each of the setof input power convertors may be replaced by a pair ofparallel-connected input power convertors 1402, 1404, to implement theabove described interleaved topology. Embodiments of this architecturealso include an interleave controller 1406 to control switching betweenrunning just one convertor of the pair and running both convertors,depending upon the power provided by a sub-string, in a similar mannerto that previously described for a complete PV panel. In this way thenumber of power convertors used for each sub-string may be variedaccording to the power from the sub-string, running both convertors whenmore power is available and just one when less power is available, oralternatively running one of the convertors designed for higher powerthan the other when more power is available from the sub-string, andrunning the other convertor when less power is available.

FIG. 15 shows a further variant circuit architecture 1500 in which theDC-to-AC conversion stages 1502, 1504, 1506 of each input powerconvertor includes both a sub-string MPPT control function, and a bypasscontrol function (in a similar manner to the architecture of FIG. 11 a).The sub-string MPPT control function provides MPPT control of the inputpower conversion stage for a sub-string according to any of a range oftechniques which will be familiar to the skilled person, but applyingthis to the sub-string rather than to the PV panel as a whole. Thebypass control function operates to bypass a shaded sub-stringresponsive to detection of a reduction in voltage and/or current and/orpower from the sub-string, for example by controlling both the switchingMOSFETs in the input DC-to-DC conversion stage to switch on and shortthe respective pair of DC input connections. The architectureillustrated in FIG. 15 includes bypass diodes which may, for example bemounted on the solar panel, and may therefore be present even if notnecessary. However we will also describe, later, arrangements in whichthese bypass diodes are omitted.

FIG. 16 a shows an implementation 1600 of a power conditioning unit inwhich the input power converter stages employ voltage-frequency basedMPPT thus the dc-to-ac power conversion stages 1602, 1604, 1606 of theinput power converters for the respective sub-strings in this exampleemploy a common reference frequency signal input 1608, 1610, 1612 withthe phases of the reference signals 60° apart (as previously described).The power transferred from the dc input to the dc link 1360 is dependenton the frequency of operation of stages 1602, 1604, 1606 which,effectively, adjusts the gain of the input power converters.

FIG. 16 b illustrates a variant system architecture 1650 of thearrangement of FIG. 16 a in which each of the input power converters1652, 1654, 1656 provides a respective dc output, and in which these dcoutputs are connected in series to provide dc power to dc link 1360,which in turn provides power to the common dc-to-ac output powerconversion stage 1658 which provides ac grid mains output 1660. In thearrangement of FIG. 16 b each input power converter includes arespective sub-string MPPT control function 1662, 1664, 1666 (incommunication with one another). Thus sub-string MPPT function mayinclude a dc gain adjust function so that if one sub-string is shadedand therefore the dc output voltage from the respective input powerconverter is reduced, the dc voltage gain of the other input powerconverters may be increased (without necessarily affecting the MPPToperating point) to compensate. In this way the voltage on dc link 1360may be maintained above the peak ac mains output voltage, for exampleabove around 170 volts or 340 volts (depending on the country), which ishelpful for implementation of the particularly advantageous powerconversion techniques we have previously described which do not useelectrolytic capacitors for energy storage.

FIG. 17 shows a further variant power conditioning unit architecture1700 in which the dc-to-dc conversion stages 1702, 1704, 1706 of therespective input power converters for the sub-strings, rather than usinga resonant converter as illustrated in FIG. 16 a, employ direct dutycycle control of the MOSFET switches to implement sub-strings MPPT. Thedirect duty cycle control comprise PWM control to control the powertransfer through an input power converter, for example based on an inputcurrent reference sensing a sub-string current (for example using acurrent sensing resistor as previously described), and a voltagereference sensing a sub-string voltage, as schematically illustrated.

FIG. 18 a illustrates a circuit architecture 1800 similar to that ofFIG. 13 a, but in which the bypass diodes are omitted. In thisarchitecture an active bypass function is provided by an MPPT/bypasscontroller 1802, as previously described. FIG. 18 b illustrates avariant system architecture 1820 to the arrangement of FIG. 18 a, inwhich the input power conversion stages 1822, 1824, 1826 each include anactive bypass control function as well as, optionally but preferably,MPPT, for example employing direct duty cycle control. The currentand/or voltage produced by a sub-string may be sensed and when this isless than a threshold value (or when these define less than a thresholdpower) shading of a sub-string may be assumed and the bypass functioncontrolled on. Alternatively shading may be detected by detecting agreater than threshold difference between the voltage and/or currentand/or power from one sub-string and a voltage and/or current and/orpower derived from one or more of the other sub-strings, for example anaverage power of all or the other sub-strings. Optionally the bypasscontrol need not entirely switch off a sub-string and, for example,additionally or alternatively to employing MPPT control for thesub-string may operate or draw power from a sub-string cyclically,operating the sub-string periodically rather than continuously to avoidoverheating. The proportion of time for which a sub-string is operatedin such a periodic manner may be dependent on the determined degree ofshading so that the sub-string may be operated for a greater proportionof the time when it is only lightly shaded, the proportion of time forwhich it operates being decreased with increased shading.

FIG. 18 c illustrates a further variant circuit architecture 1840 ofFIG. 18 b, illustrating further circuit details, and the provision ofMPPT control for each of power conversion stages 1822, 1824, 1826. Therespective controllers 1828, 1830, 1832 for each sub-string (which may,in embodiments, be implemented on a common, shared microcontroller insoftware and/or hardware) each have a mode control input 1834, 1836,1838, in embodiments implemented as a connection to the gate drivecircuits of the respective MOSFETs. This control input may be employedby a separate bypass controller 1842 to control the gate drives to theMOSFETs of a sub-string to switch the MOSFETs on. Alternatively thebypass controller 1842 may be integrated with the controllers 1828,1830, 1832. With either implementation, as previously described, broadlyspeaking the bypass control function enables the power conditioning unitto extract as much of the power as is available per sub-string, up tothe point where the current source (of a sub-string—see FIG. 10) willreverse. At this point the sub-string is bypassed to in effect,re-circulate this energy. This provides substantially more control overpart of the panel behaviour. As previously described, the bypass controlfunction may be combined with MPPT control.

The circuit implementation of FIG. 18 c also shows some further detailsof preferred implementations, in particular showing current sensing byvoltage sensing across current sensing resistors 1342, 1344, 1346 foreach separate sub-string, and output inductors 1846, 1848, 1850 for eachof the respective input power converters. The output inductors, eachconnected in series between the dc output of an input power converterand the dc link 1360 provide proportional load sharing between the inputpower converters/sub-strings. This proportional load sharing may also beapplied as a preferred feature in any of the previously describedcircuits/circuit architectures.

FIG. 19 a shows a block diagram of a sensing arrangement for thearchitecture of FIG. 18, illustrating a module 1900 which providesvoltage and current sensing for each sub-string, and which provides amode control output for each sub-string to operate the bypass controlwhen necessary. FIG. 19 b illustrates a similar system in which thepreviously described full bridge output stages of the input convertersare placed by respective synchronous rectification stages 1902, 1904,1906, for reduced losses. Again these synchronous rectifiers may also beemployed in any of the previously described embodiments. FIG. 19 c showsa preferred embodiment of a sub-string voltage sensing circuit 1910,which may also be used for current sensing by sensing the voltage acrossa current sense resistor. The circuit shown is able to measure a‘floating’ voltage and can also provide a significant voltage gainwithout the need for an operational amplifier. Thus the circuitcomprises a full bridge rectifier 1912 coupled between first and secondvoltage sense lines 1914, 1916, which provides an output to aprogrammable current source 1918, for example comprising a transistor.This in turn provides a current input to a current mirror 1920 whichprovides a current output on line 1922 which is converted to a voltageby resistor 1924 to provide an output on line 1926, the outputcomprising a voltage proportional to the voltage across voltage sensinglines 1914, 1916. Current mirror 1920 provides gain in a circuit. Wherenecessary mirror 1920 may be provided with multiple mirrored currentoutputs.

FIG. 20 shows a schematic block diagram of a multi purpose photovoltaicpower conditioning unit circuit board 2000. The illustrated architectureis similar to that of FIG. 18 but the technique at FIG. 20 may beemployed with the other architectures described. In the circuit board ofFIG. 20 each input power converter 1822, 1824, 1826 is provided with apair of dc input connections 1862 a, b, 1864 a, b, 1866 a, b each with arespective termination 1872 a, b, 1874 a, b, 1876 a, b. These may eitherbe each connected to a respective sub-string as illustrated by thedashed lines, in the manner of FIG. 11 a or FIG. 11 b. Alternativelythese may be connected in parallel to provide a common pair of dc inputconnectors for two or more input power converters which is thenconnected the complete PV panel, to provide a topology of the typeillustrated in FIG. 4 a. The controller 2002 controls each of the inputpower converters and comprises software or firmware to implement eitherthe interleaved control strategy described above for a complete PVpanel, switching input power converters in/out as needed, according tothe power from the panel, or any necessary sub-string level control. Ina simple embodiment no sub-string control is needed as, for example,MPPT control is provided by an MPPT control loop operating on the commonoutput stage of the power conditioning unit. However microcontroller2002 may optionally provide separate, sub-string MPPT control and/orbypass control. The operational mode of the circuit board may beselected, for example, by selecting a link on the circuit board and/orby writing a register value or modifying the firmware. The describedarrangement is useful because a similar circuit board can be employed inmultiple different operational modes depending merely on how the inputconnections are made, with a software/firmware set up for the desiredapplication—either sub-string—power conversion or ‘interleaved’ powerconversion.

FIG. 21 shows a photovoltaic panel system 2100 comprising a PV panel2102 in combination with a power conditioning unit 2104, for exampleaccording to any of the previously described architectures. In the PVpanel system of FIG. 21 the power conditioning unit 2104 is integratedwith the panel in that a circuit board 2104 of the power conditioningunit is connected directly to each of a set of sub-string tabs 2106 a-ddefining sub-strings of the PV Panel. Furthermore the circuit boards2104 include the bypass diodes or, more preferably, these may be omittedand an active bypass function provided the board 2104 may be mounteddirectly on the panel by means of the electrical connections to thesubstrings, for example by soldering the board directly to the tabs orby clipping onto the tabs (to ease thermal stresses). Preferably thepower conditioning unit is enclosed within a sealed enclosure 2108,environmentally sealing the panel and power conditioning unit together.The combined panel and power conditioning unit may then directly providean ac mains power supply output, for example via a lead 2110 andoptional connector 2112. The PV panel system including the powerconditioning unit preferably includes a heat sync 2114 on the enclosure2108, preferably opposite the panel and in thermal contact (not shown)with the circuit board 2104. Preferably a region of thermalinsulation/separation 2116 is included between the power conditioningunit circuit board 2104 and the PV panel itself 2102.

Continuing to refer to FIG. 21 and to integration of the powerconversion unit and PV panel, these techniques enable a system where theonly cable or cables from the panel carry the AC grid voltage/currentand not the PV voltage/current. This is thus a fully integrated solutionwhich, in embodiments, results in the elimination of the bypass diodescreating a true AC solar module. There is also no need for a solarjunction box specifically for the tabs that emerge from the back of asolar module as the micro-inverter is soldered (for example by a wavesolder process) or connected directly to these (for example, by one ormore screw connectors). Thus the micro-inverter is effectivelyincorporated into the junction box and obviates the need for bypassdiodes.

In summary, power harvest from photovoltaic power sources is subject tosome real limitations of panel structure and behaviour. This is true forall PV technologies where cell variance and performance under shadinglowers the overall harvest potential of the panel. The advent of DCoptimizers and micro-Inverters allows for additional local monitoringand conversion, offering a higher level of control of panel performanceand behaviour of the photovoltaic panels. The techniques we havedescribed allow for a single MPPT algorithm sensed on the grid side ofthe converter. This MPPT technique is safe and simple and facilitatesthe sub-string converter approach we have described for both paralleland series connected PV sources. Some features and advantages ofpreferred embodiments and aspects of the invention (which may beprovided in any combination) include:

-   -   1. Sub-string conversion in the range 6-12V D.C.    -   2. Interleaved operation with variable phase control over power        range; lower ripple on the output stage; and three phase        operation 60/120/180 phase operation of each stage.    -   3. Lower power conversion with lower current processing.    -   4. Lower voltages allowing low voltage MOSFETs to be used, for        lower loss and higher reliability.    -   5. Individual or separate conversion stages.    -   6. Integration of a three phase magnetic structure, resulting in        higher efficiency.    -   7. Novel voltage to frequency adjustment of dc transformer        performance, on and off resonance with, potentially, 1.5 voltage        amplification at lower power levels.    -   8. Conversion efficiency in the order of 98.5%-99%.    -   9. Active bypassing using an ideal bypass diode (an enhanced        MOSFET), leading to higher reliability and lower loss.    -   10. Frequency control and/or direct duty cycle control of        conversion stage.    -   11. Floating voltage sensing which can be referenced to PV +ve        or −ve. This can be useful, inter alia, in +ve grounded PV        systems.    -   12. Transformer driven synchronous rectification with an LLC        converter, with low power operation using the body diode.

No doubt many other effective alternatives will occur to the skilledperson. It will be understood that the invention is not limited to thedescribed embodiments and encompasses modifications apparent to thoseskilled in the art lying within the scope of the claims appended hereto.

I claim:
 1. A photovoltaic (PV) power conditioning unit for deliveringpower from a PV panel to an ac power supply output, the PV panelcomprising a string of series-connected solar cells with at least oneelectrical tap connection on said string to define a set ofseries-connected sub-strings of said solar cells, the PV powerconditioning unit comprising: a plurality of input power converters, onefor each said sub-string, each input power converter having a pair of dcinput connections for connection to either end of a sub-string in theset of sub-strings, each of said input power converters having a poweroutput, wherein each electrical tap connection on said string connectsto a dc input connection of each of two power converters in theplurality of input power converters; a sub-string sensing systemcomprising a voltage-programmed current source having a set of currentprogramming inputs, each current programming input to receive a voltagesignal from a sub-string in the set of sub-strings and having acorresponding programmed current output to output a programmed currentdependent on a level of said voltage signal; at least one input powerconverter controller coupled to said sub-string sensing system tocontrol a level of power conversion of one or more input powerconverters responsive to a voltage from a particular sub-string receivedby the sub-string sensing system, wherein said programmed current outputcorresponding to the particular sub-string is coupled to acurrent-to-voltage converter to convert said programmed current to avoltage dependent on said programmed current for input to said inputpower converter controller; a shared dc link to provide a common dc busfor said plurality of input power converters, wherein said power outputof each said input power converter is coupled to said shared dc link toprovide power from a sub-string in the set of sub-strings to said dcbus; an energy storage capacitor, coupled to said shared dc link, tostore power from said plurality of input power converters for deliveringto said ac power supply output; and a common output power conversionstage coupled to said shared dc link to convert dc power from saidshared dc link to ac power to provide an ac output from said PV powerconditioning unit.
 2. The photovoltaic power conditioning unit of claim1, wherein each input power converter in the plurality of input powerconverters is a voltage-increasing dc-to-dc power converter.
 3. Thephotovoltaic power conditioning unit of claim 2, wherein each inputpower converter in the plurality of input power converters comprises adc-to-ac converter, wherein said dc-to-ac converters of said input powerconverters for said sub-strings are synchronized.
 4. The photovoltaicpower conditioning unit of claim 3, wherein said dc-to-ac convertersdrive a shared power coupling transformer, wherein said power output ofan input power converter comprises a winding of said shared powercoupling transformer, wherein each said input power converter is coupledto said shared dc link by a common output winding of said shared powercoupling transformer.
 5. The photovoltaic power conditioning unit ofclaim 4, wherein the common output winding of the shared power couplingtransformer provides power to a shared rectification stage.
 6. Thephotovoltaic power conditioning unit of claim 1 further comprising: asensor to sense one or both of a voltage on a sub-string and a currentprovided by said sub-string; and a bypass controller coupled to saidsensor to detect shading of said sub-string and, responsive to saiddetection, to control an input power converter connected to said shadedsub-string to reduce or stop power conversion by said input powerconverter from said shaded sub-string.
 7. The photovoltaic powerconditioning unit of claim 6, wherein said input power converterconnected to said sub-string comprises a dc-to-ac converter comprising aset of switches, wherein said bypass controller is configured to controlsaid switches to switch on, responsive to detection of said shading, toprovide a bypass current path for said shaded sub-string through theinput power converter for the shaded sub-string.
 8. The photovoltaicpower conditioning unit of claim 6, wherein said power outputs of saidinput power converters are connected in series, wherein said bypasscontroller is configured to increase a voltage gain of one or moreunshaded said input power converters to compensate for said reduced orstopped power conversion from said shaded sub-string.
 9. Thephotovoltaic power conditioning unit of claim 1 further comprising anoutput power converter maximum power point tracking (MPPT) controlsystem, wherein said output power converter MPPT control system isconfigured to control said common output power conversion stage tomaximise maximize said dc power drawn from said shared dc link.
 10. Thephotovoltaic power conditioning unit of claim 1, wherein each of saidinput power converters has a respective associated sub-string maximumpower point tracking (MPPT) controller to provide separate MPPT controlfor each sub-string of said PV panel.
 11. The photovoltaic powerconditioning unit of claim 10 further comprising: a sensor to sense oneor both of a voltage on a sub-string and a current provided by saidsub-string; and a bypass controller coupled to said sensor to detectshading of said sub-string and, responsive to said detection, to controlan input power converter connected to said shaded sub-string to reduceor stop power conversion by said input power converter from said shadedsub-string, wherein a sub-string MPPT controller of said sub-stringincludes said bypass controller.
 12. The photovoltaic power conditioningunit of claim 1, wherein each said input power converter comprises: aset of power converters having respective inputs and outputs connectedin parallel; and a power level controller to selectively enableoperation of power converters of said set of power converters responsiveto a detected level of power being drawn from a corresponding sub-stringto which the set of power converters is connected.
 13. The photovoltaicpower conditioning unit of claim 1, wherein the power outputs of theinput power converters are connected in parallel to the shared dc link.14. A photovoltaic (PV) power conditioning unit for delivering powerfrom a PV panel to an ac power supply output, the PV panel comprising astring of series-connected solar cells with at least one electrical tapconnection on said string to define a set of series-connectedsub-strings of said solar cells, the power conditioning unit comprising:a set of input power converters, one for each said sub-string, eachinput power converter having a pair of dc input connections forconnection to either end of each of two sub-strings, each of said inputpower converters having a power output; a shared dc link to provide acommon dc bus for said set of input power converters, wherein said poweroutput of each said input power converters is coupled to said shared dclink to provide power from a sub-string in the set of sub-strings tosaid dc bus, wherein each input power converter comprises a dc-to-acconverter, wherein said dc-to-ac converters of said input powerconverters for said sub-strings are synchronized such that each operatesat a successive relative phase offset of (180°/n) or (360°/n) to reducea voltage ripple on said dc link, wherein n is a number of the inputpower converters; an energy storage capacitor, coupled to said shared dclink, to store power from said PV panel for delivering to said ac powersupply output; and a common output power conversion stage coupled tosaid shared dc link to convert dc power from said shared dc link to acpower to provide an ac output from said PV power conditioning unit. 15.A photovoltaic (PV) power conditioning unit for delivering power from aPV panel to an ac power supply output, the PV panel comprising a stringof series-connected solar cells with at least one electrical tapconnection on said string to define a set of series-connectedsub-strings of said solar cells, the power conditioning unit comprising:a sensing circuit to sense a voltage or current from one or more of saidsolar cells, said sensing circuit comprising: a sense input to receive avoltage signal from said one or more solar cells dependent on saidsensed voltage or current; and a voltage-programmed current sourcehaving a current programming input coupled to said sense input, andhaving a programmed current output to output a programmed currentdependent on a level of said voltage signal, wherein said programmedcurrent output is coupled to a current-to-voltage converter to convertsaid programmed current to a voltage output from the sensing circuitdependent on said programmed current.
 16. The photovoltaic powerconditioning unit of claim 15, wherein said sense input is coupled tosaid current programming input of said voltage-programmed current sourcevia a full-wave rectification circuit.
 17. The photovoltaic powerconditioning unit of claim 15 further comprising a current minor coupledbetween said programmed current output of said voltage-programmedcurrent source and said current-to-voltage converter.